1. Field of the Invention
The invention relates to an FM demodulator having a first and a second input terminal for receiving FM modulated input signals having a 90.degree. degree phase relation, and an output terminal for supplying a demodulated FM signal, said FM demodulator comprising:
a phase comparator having a first and second input and an output, said first input being coupled to the first input terminal of said FM demodulator and said output being coupled to the output terminal of said FM demodulator. PA1 a first tunable phase shifting means coupled between the second input terminal of said FM demodulator and the second input of said phase comparator, said first turnable phase shifting means comprising a tuning control input for controlling a tuning frequency of said first phase shifting means, PA1 a feedback path from the output of said phase comparator to said tuning control input, comprising a loop filter.
The invention also relates to a receiver for receiving FM signals, comprising an RF stage, an IF stage, an FM demodulator and an LF stage.
2. Description of the Related Art
An FM demodulator as described in the opening paragraph is known from the European Patent Application EP-A 574, corresponding to U.S. Pat. No. 5,341,107. In this patent, an FM demodulator is disclosed having a first and a second input terminal and an output terminal. To these input terminals, FM modulated input signals are applied, having a 90.degree. phase relation. In the known FM demodulator, first and second phase shifting means are combined in a polyphase filter having in-phase and quadrature inputs and outputs, and a tuning control input for controlling a tuning frequency of said polyphase filter. The in-phase and quadrature inputs of said polyphase filter are coupled to the first and second input terminals, respectively, and the in-phase and quadrature outputs of the polyphase filter are coupled to second inputs of second and first phase comparators, respectively. Said first and second phase comparators further comprise first inputs, said first inputs being coupled to said first and second input terminals, respectively. Outputs of the first and second phase comparators are coupled to a subtracting stage, an output of said subtracting stage being coupled via a feedback path to the tuning control input of said polyphase filter. Through this feedback path, the tuning frequency of the polyphase filter will track the instantaneous frequency of the FM input signal. Thus, a tuning control loop is realized. By deleting the second phase shifting means, the second phase comparator and the subtracting stage, an asymmetrical FM demodulator is obtained, as these components are not essential to the FM demodulation function per se. In this asymmetrical FM demodulator, a tuning control loop can be identified, comprising the phase shifting means, the phase comparator, and the loop filter. The FM threshold of the FM demodulator is a.o. determined by the bandwidth of the phase shifting means. Thus, a reduction of the bandwidth of the phase shifting means results in a reduced FM threshold. However, such a reduction can lead to instability in the tuning control loop, which makes the known FM demodulator unsuitable for practical use.